This invention relates generally to power generation system and, more particularly, to a heat recovery system in cogeneration plants.
Cogeneration plants often employ a duct burner placed in the turbine exhaust gas (TEG) upstream of a heat recovery steam generator (HRSG). The duct burner is disposed in a duct connecting the gas turbine to the HRSG. The fuel injected by the duct burner is oxidized by the oxygen present in the TEG stream. The use of the duct burner increases the overall capacity of the steam output from the HRSG and adds operational flexibility of the heat recovery system.
In a typical configuration of the duct burner, several horizontal lines of firing are provided by duct burner elements that are spaced uniformly in a vertical direction. Each duct burner element includes a gas header with flame stabilizers attached, or disposed adjacent, to the header. The duct burner assembly often includes baffles positioned between the duct burner elements. The primary function of the baffles is to increase the flow velocity around the duct burner elements. This increase in velocity is desirable, and may be necessary, for achieving the desired flame intensity and completeness of combustion. A secondary function of the baffles is to create an additional resistance that helps balance the TEG flow. Sometimes the baffles are used as structural members that via linkages support the duct burner elements. In a large system, the duct may have a cross section of about 30 feet wide by 50 feet high at the location of the duct burner. The duct burner may be composed of eight to twelve duct burner elements and a large number of baffles between the elements.
The duct connecting the gas turbine to the HRSG typically expands in cross-section with a steep slope at the top. The TEG flow has a very high level of turbulence and usually is poorly distributed across the duct. In order for the duct burner to perform effectively, improvements in the flow distribution are desired, especially in the vertical direction. In many cases the desired flow uniformity is achieved with the help of a distribution grid constructed across the duct. The grid performs a flow balancing action. Such grids are quite expensive as they are made of high grade stainless steel. The presence of the grid also causes detrimental pressure losses in the duct that reduce the efficiency of the turbine.
In some cases the HRSG includes a tubular superheater (SH) having an upstream SH section upstream of the duct burner and a downstream SH section downstream of the duct burner. Having a section of the SH unaffected by the duct burner has several benefits. The upstream SH section helps provide better control of steam temperature over the load range and lower peak temperatures of the SH tubes. The upstream SH section also helps improve the TEG flow distribution at the duct burner and allows the distribution grid to be eliminated or simplified. The superheater typically includes a plurality of SH tubes that are usually vertically disposed. The SH tubes may include fins extending from their surfaces into the TEG flow to increase heat extraction. Horizontal SH tubes may be used, but they are generally more difficult to support than vertical SH tubes.
Advances in gas turbine technology result in lower temperature and lower O2 concentration in the turbine exhaust gas, placing greater demands on the duct burner. Extraction of heat from the TEG upstream of the duct burner further reduces the TEG temperature and exacerbates the problem. The use of the upstream SH section thus has a serious drawback by lowering the TEG temperature upstream of the duct burner. The lower TEG temperature coupled with lower oxygen concentration negatively impacts the duct burner performance (e.g., lower combustion efficiency, higher carbon monoxide and unburned hydrocarbon emissions). This typically necessitates the use of a more complicated and costly duct burner. In some cases the injection of fresh air (augmenting air) in the duct burner is needed for the duct burner operation, which can significantly increase burner cost and adversely affect the overall efficiency. Increased TEG temperature in the area of the duct burner flame would improve the duct burner performance and allow the use of simpler duct burners.
The present invention is directed to a heat recovery system in which the superheater and duct burner are arranged to provide improved performance. In preferred embodiments, the heat recovery system is a heat recovery steam generator (HRSG) including a superheater (SH) with an upstream SH section and a downstream SH section. The SH sections and duct burner are arranged to reduce the negative effects of heat extraction by the upstream SH section on the duct burner operation. The upstream SH section is arranged to provide a downstream flow that is temperature stratified and position the duct burner elements in the areas of maximum temperature, which may be as high as the temperature of the TEG upstream of the upstream SH section.
In one embodiment, the upstream SH section is disposed in the upper portion of the TEG duct connecting the gas turbine to the HRSG, while the duct burner elements are clustered in the lower portion of the duct. In this way, the duct burner elements are largely unaffected by the heat extraction from the TEG stream in the upper portion by the upstream SH section.
In another embodiment, the duct burner elements and the SH tubes of the upstream SH section are vertically disposed and alternately arranged in the horizontal direction to produce horizontal temperature stratification. In yet another embodiment, the duct burner elements and the SH tubes of the upstream SH section are horizontally disposed and alternately arranged in the vertical direction to produce vertical temperature stratification. In these arrangements, the effects of heat extraction from the TEG stream on the operation of the duct burner elements can be eliminated, or significantly reduced.
In yet another embodiment, the SH tubes of the upstream SH section are vertically disposed and the duct burner elements are horizontally disposed. The SH tubes have variable heat absorption in the vertical direction such that the heat extraction is lower adjacent the horizontally disposed duct burner elements, desirably much lower, than the heat extraction in regions between the duct burner elements. In this way, the temperature drop in the portion of the TEG flow adjacent the duct burner elements is minimized. Of course, an alternate configuration may employ horizontally disposed upstream SH tubes and vertically disposed duct burner elements.
In accordance with an aspect of the present invention, a heat recovery apparatus comprises a duct for receiving a flow of fluid in a flow direction. A heat extraction member is disposed inside the duct, and includes heat extraction elements for extracting heat from the flow of fluid. The heat extraction elements cover a heat extraction portion of a heat extraction cross section of the duct on which the heat extraction member is disposed. A duct burner is disposed inside the duct. The duct burner comprises duct burner elements disposed in a duct burner portion of a duct burner cross section of the duct on which the duct burner is disposed. The heat extraction portion when projected in the flow direction onto the duct burner cross section substantially does not overlap with the duct burner portion. The duct burner cross section is disposed downstream of (preferably immediately downstream of), or at the heat extraction cross section.